gdbstub: fixes cases where wrong threads were reported to GDB on SIGINT

gdbstub: fixes cases where wrong threads were reported to GDB on SIGINT

This fix is implemented by having the vCont handler set the value of
`gdbserver_state.c_cpu` if any threads are to be resumed. The specific
CPU picked is arbitrarily from the ones to be resumed, but it should
be okay, as all GDB cares about is that it is a resumed thread.

Signed-off-by: Matheus Branco Borella <dark.ryu.550@gmail.com>
Message-Id: <20230804182633.47300-2-dark.ryu.550@gmail.com>
[AJB: style and whitespace fixes]
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1725
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230829161528.2707696-9-alex.bennee@linaro.org>